All sorts of natural stones make great garden decor! They don't have to be huge to work, either. Above are a few of the crystal friends I keep between two trees in our yard. Smaller rough stones or natural clusters make great accent pieces around smaller plants or in planted containers.
Make sure your outdoor crystals are safe outside- Quartz varieties such as Amethyst, Smoky Quartz, Aventurine etc, and many other stones are safe to leave outside but some stones such as Selenite, can not get wet. (Selenite will dissolve if left outside.)